Electric brain stimulation has been shown to be as potentially effective treatment for a number of brain disorders, including epilepsy, migraine, fibromyalgia, major depression, stroke rehabilitation, and Parkinson's disease, and is also used in electrocorticography and Cortical Stimulation Mapping (CSM). In epilepsy, the generally accepted treatment method involves locating the epileptic focus in the brain, which is found using EEG analysis of epileptiform discharges and resultant spike or seizure voltage fields at the scalp. The temporal lobe is often the location of epileptogenesis, though the position and orientation may vary between individuals. In treating major depression, the target is often the left dorsolateral prefrontal cortex. In treating migraine, the visual cortex or motor cortexes are generally targeted, as well as the occipital nerves. In stroke recovery for reduced limb movement, the area of the motor cortex associated with the limb is the usual target. Once the target site is located, electrical stimulation is applied to provide therapy for the specific indication.
Electric brain stimulation may be accomplished by several means. Repetitive Transcranial Magnetic Stimulation (TMS) is a noninvasive technique that uses a coil to deliver a series of high energy magnetic pulses to the brain, thereby inducing current to flow in the cortex underneath the coil. rTMS has been shown to be effective in the treatment of major depression, and other mental disorders. However, it is not easily directed to a particular location, and involves a large, expensive device to generate the high current pulse to the coil. rTMS is not portable and requires a treatment administrator to deliver therapy to the patient.
Transcranial Direct Current Stimulation (tDCS) uses electrodes on the outside of the head to deliver small amounts of current to the brain. tDCS was originally used for stroke recovery, and has shown promise in the treatment of some mental disorders and for cognitive improvement. Electrodes are located near the region of interest for stimulation. The vast majority of current is shunted between the electrodes since the skull is a very effective electrical insulator. However, a portion of the current does result in intracerebral current flow, which may increase or decrease neuronal excitability and alter brain function. The exact method of action is unclear. tDCS current strength is limited due to the excitability of nerves in the scalp, which can cause discomfort to the patient if the current is set too high.
Vagus nerve stimulation involves electrically stimulating the vagus nerve in the neck of the patient. This can be done either using electrodes on the skin, which may involve painful sensation of the patient, or surgically implanting electrodes near the vagus nerve, generally with a power source implanted elsewhere in the body. This involves a significant surgical procedure and has shown efficacy in treatment of epilepsy and depression.
Deep brain stimulation (DBS) uses electrodes implanted and placed bilaterally into the basal ganglia, cerebellum, anterior principal nucleus, the centromedian nucleus, caudate nucleus, thalamic, or subthalamic region. Stimulation may also be delivered subcortically. Stimulus trains are delivered for treatment of a number of disorders, including epilepsy, Parkinson's disease, and major depression. DBS is generally a very invasive procedure, requiring a long lead that penetrates the skull with multiple electrodes near the tip. The procedure is considered major surgery and is not generally used unless other methods have been exhausted.
Direct cortical stimulation (DCS) is similar to DBS, except that the lead lies on the surface of the cortex, either subdural or epidural. The location of the electrodes is generally near the seizure foci. The electrodes are secured in place using sutures. This technique often involves removing a portion of the skull to gain access to the cortical surface, and possibly to make room for the power source. DCS has been shown to have efficacy in treatment of epilepsy and neuropathic pain.
It is possible to perform electrical stimulation of the brain utilizing a current loop through a conductive path involving probes that penetrate the skull at two or more points. Since the skull is highly resistive (approximately 80 times more resistive than the cerebrospinal fluid (CSF) around the brain), it acts as a good insulator. If the area around the two probes that penetrate the skull is filled with a resistive material, like silicone, then a current source through one probe would not have a low impedance return path except through the other probe. Each probe could have an electrode at the tip, and the length, angle, and position of each probe would allow for precise stimulation of any area of the brain. A second pair of electrodes could be located subcutaneously, so that the current return path would travel through or under the scalp. The current source could be an implanted pulse generator, an ultrasonic transducer, or a coil that uses an external alternating magnetic field to induce electric current. Other current sources are also possible as well. If magnetic induction is used, the external portion could be a small case that contains a coil or rotating magnet. This would likely be not much larger than a cigarette pack and would be very light. In the case of a rotating magnet, a diametrically magnetized cylindrical magnet rotating at 500 Hz can induce a 10 mA p-p current in a 600-turn coil through a 2000 ohm load. This configuration has advantages over existing devices and methods. It allows for more precise stimulation than rTMS and the external portion is smaller and more portable. It allows for more precise stimulation than tDCS with greater stimulation current. It allows for more precise stimulation than Vagus nerve stimulation, with better understood method of action.
To implant the implantable portions, up to two small incisions are made in the scalp and small burr holes drilled in the skull (similar to trephination) at locations on opposite sides of the intended stimulation site. The probes are inserted through the burr holes. The probes contain a resistive material to fill the space in each burr hole. Both implants are pressed flat. Impedance testing or stimulation may be performed to ensure proper placement of the subcranial electrodes. The scalp is sutured and the procedure is complete. Such a procedure could be performed on an out-patient basis, with a mild anesthetic. By contrast, DCS generally involves a significant craniotomy, and the electrodes must be sutured in place to ensure migration does not occur. No electrode sutures are required for the current loop procedure, since the probes are held in place in the burr holes. DBS is a much more invasive procedure in which a lead contains multiple electrodes, and stimulation occurs between two or more of those electrodes to affect a particular region of the brain. In the present invention, current flows between the top of two probes, thereby potentially covering a larger area.
U.S. Patent Application No. 2011/0218588 (Jung) is directed at electrical stimulation of the cortex with an externally powered magnetic coil and one or more implants with coils that are powered inductively. For as single implant, a plug with 2 electrodes at the tip is used. For a pair of implants, the induced current flows from one implant, through the cortex, through the other implant and back through a wire. By specifying a wire between the two implants, this patent points away from applications such as the current application, which makes use of the skull's high impedance and the low impedance of the scalp to form the current path without the necessity of a wire. Since the present invention does not use a subcutaneous wire to connect the implants, it represents a novel approach to the stimulation technique, and has significant advantages over prior art, since a wire would need to be tunneled under the skin to connect the two implants, requiring additional time in surgery. In addition, current through the scalp may provide the patient with feedback (e.g., a tingling sensation), indicating that current is flowing and stimulation is being administered.
U.S. Patent Application No. 2011/0112602 and 2011/0046693 (Lee) are both directed at using an external magnetic field to induce a current in a subcutaneous coil, with a long probe that extends into the cerebral nerve of the patient to provide DBS. The patents are specific to DBS and only provide for a single probe to proceed through the skull. Since the present invention provides for two or more probes, it is a novel approach that allows for stimulation of a greater number of locations either at the cortex (if the probes are short and just touch the cortex), or deeper in the brain (if the probes are longer and extend into the brain). This has significant advantages over the prior art, since the stimulation site may cover a wider area than a single probe would allow, and two probes allow for better control of the stimulation, ensuring that current proceeds through the brain and not along the probe from one electrode to the other.
U.S. Pat. No. 6,205,359 (Boveja) is directed at treating epilepsy using an implantable coil and lead that is electrically attached to the Vagus nerve. Stimulation is achieved by activating a coil above the surface of the skin which induces current in the implantable coil that provides electric current for stimulation. This patent is directed at Vagus nerve stimulation. It does not mention direct stimulation of the brain. The stimulation provided by the present invention represents a novel approach that has advantages over Vagus nerve stimulation, since the current is applied to the cortical site more directly, with a better understood method of action.
U.S. Patent Application No. 2010/0324623 is directed at tDCS in which probes pierce the skull to provide direct stimulation to the cortex, removing the skull impedance from the path, allowing for greater stimulation current. This patent uses two probes that are stimulated electrically. The probes have an external portion, outside the skin, and an internal portion that proceeds to the cortex. The present invention is a novel approach in that it removes the skull's impedance from the current path, and actually uses the skull's high impedance to create a current loop, allowing stimulation current to be directed at a location in the brain. The present invention has an advantage over the prior art in that it is less invasive, with less pain to the patient, and less chance of infection.